1. Field of the Invention
The present invention relates to a developing device for use in an image forming apparatus such as a copying machine, a facsimile, a printer, or other similar image forming apparatuses, and more particularly to a developing device that develops a latent image formed on an image bearing member with a two-component developer, i.e., a mixture of toner and magnetic carrier particles.
2. Discussion of the Background
In a background developing device that develops a latent image formed on an image bearing member with a two-component developer containing a mixture of toner and magnetic carrier particles, it is known that a toner density in the two-component developer is controlled by the movement of the developer without using a toner density detecting device.
For example, Japanese Laid-open Patent Publication No. 63-4280 describes a developing device which includes; a developer container with an opening for containing a developer containing a mixture of toner and magnetic carrier particles; a developer carrying member having a magnetic field generating device therein, disposed in the opening of the developer container, for carrying the developer out of the developer container to a developing position where a latent image is developed with the developer; a developer regulating device spaced apart from a surface of the developer carrying member for regulating a thickness of a developer layer; and a developer movement limiting member which is mounted to an inside surface of the developer regulating device and limits the moving region of the developer in the developer container. In addition, a developer layer formed adjacent the surface of the developer carrying member is formed into a movable layer which moves following the movement of the surface of the developer carrying member and into a stationary layer which is formed outside of the movable layer and substantially unmovable by being stopped by the limiting member. Outside the stationary layer, there is formed a toner layer containing substantially only toner particles. The stationary layer deforms along a magnetic line of force that is produced by the magnetic field generating device.
The above-described background developing device controls the toner density in the developer on the basis of the movement of the developer itself and eliminates the need for a toner density control mechanism including a developer agitating and conveying member. This successfully reduces the size and cost of the developing device.
In the developing device with the above-described configuration, the movement of the developer effecting the supply of toner depends on an amount of toner on the surfaces of the developer, that is a toner covering ratio of the developer. Specifically, when the toner covering ratio of the developer at a position upstream of the developer regulating member in a developer conveying direction is low, the developer is quickly circulated and a supply of toner is enhanced. As the toner covering ratio of the developer is higher, the developer is circulated slowly. When the toner covering ratio is approximately 100%, the supply of toner is stopped. A condition in which toner particles fully cover the surface of a single carrier particle in a single layer without any clearance represents a 100% toner covering ratio.
In the above-described developing device, an exposed area of the developer which contacts the toner supplied from the developer container is relatively large, and the amount of supply of toner depends on how the developer is carried by the developer carrying member. Specifically, when the developer is unevenly deposited on the surface of the developer carrying member, the toner is irregularly supplied to the developer on the developer carrying member. For example, at a place where much developer is carried on the surface of the developer carrying member, the toner is not positively supplied to the developer. On the other hand, the toner is positively supplied to the developer at a place where little developer is carried on the surface of the developer carrying member. As a result, an irregular toner density occurs on an image. Further, in the developer conveying direction of the developer carrying member, the toner is not supplied to a place apart from the surface of the developer carrying member where the developer moving speed is about 0 mm/sec. On the other hand, the toner is supplied to a place near the surface of the developer carrying member where the developer moving speed is higher. In the above-described condition, the supply of toner is not precisely controlled, so that the toner density is not accurately controlled.
The irregular toner density causes a background fouling when the toner density is high, and a lower image density or carrier adhesion to the image when the toner density is low.
Another background developing device includes a developer carrier having a magnetic field generating device therein, configured to carry and convey a developer containing toner and magnetic carrier particles; a first regulating member configured to regulate the developer being carried and conveyed by the developer carrier; a developer storing section configured to store developer scraped off the developer carrier by the first regulating member; and a toner storing section provided adjacent to the developer storing section to supply toner to the developer carrier. Further, the developer storing section includes a second regulating member arranged upstream of the first regulating member with respect to a direction in which the developer carrier conveys the developer. The second regulating member is spaced from the developer carrier such that when a thickness of a layer of the developer on the developer carrier increases due to an increase of the toner density in the developer on the developer carrier, and regulates an increased amount of the developer being carried and conveyed by the developer carrier. A contact condition of the developer carried on the developer carrier with the toner supplied to the developer carrier from the toner storing section is varied in accordance with variation of a toner density in the developer carried on the developer carrier to thereby vary a condition of the developer on the developer carrier to attract the toner.
In the above-described background developing device, the toner and magnetic carrier particles are agitated by circulation of the developer in the developer storing section. This successfully avoids the charging amount of toner from lowering, and reduces the occurrence of deterioration of image quality, such as, an irregular image density and a fog of an image. In the developing device with the above-described configuration, in order to further reduce the occurrence of an irregular image density and a fog of an image, active circulation of the developer in the developer storing section is desired.